Vapor compression refrigeration systems are well known in the art. Vapor compression systems circulate refrigerant to achieve a desired cooling effect. Starting with the evaporator, which is where the desired cooling effect is achieved, refrigerant evaporates, thereby carrying away the latent heat of vaporization, as it passes through the evaporator. The evaporator serves as a heat exchanger. Air, water, glycol or other cooling mediums, which are hereinafter referred to interchangeably as “water” or “secondary fluids,” forced over the surface of the evaporator are cooled. At the same time, the evaporator surface is warmed, thereby continuing the vaporization of the refrigerant.
Refrigerant leaving the evaporator is in a low pressure gas state. This low pressure refrigerant is then compressed by a compressor to a hot, highly pressurized gas state and passed to yet another heat exchanger, the condenser, which is normally outside, or a distance away from, the object or building being cooled. The condenser provides a location wherein the hot refrigerant is permitted to condense to a liquid. This results in heat transfer from the refrigerant to cooler surroundings. After passing through the condenser, the liquid refrigerant enters the expansion valve where it again expands to the evaporator pressure.
As most people are aware, air conditioning is used to cool buildings in the warm summer months. However, many buildings also require mechanical cooling during the winter months as well. This is due to high internal heat loads that are generated by processes, persons, lighting, computers and other office equipment situated within the building. Other buildings that have high internal heat loads often house factory equipment. If the building has no means to bring in outside air to cool the building, then energy intensive mechanical cooling systems must be operated.
Typical mechanical cooling systems are generally designed to run during the warmer months of the year. If a mechanical cooling system is required to operate during very cold weather, the system will often fail. Such failures normally occur at temperatures below 32 degrees Fahrenheit. In general, this failure is caused by the refrigerant having a tendency to migrate to the coldest spot in the cooling system, normally the condenser, which is located outside. As a result, the compressor starves for refrigerant and is automatically shut down on low evaporator pressure.
One way to keep the compressor running in the foregoing situation is to employ a liquid refrigerant pump after the condenser to pump liquid refrigerant to the expansion device. The use of a refrigerant pump as described will also eliminate flash gas in the outlet lines leading to the expansion valve. This permits the compressor to run at lower discharge pressures and temperatures, thereby saving energy by taking the load off the compressor. However, such methods may not be as efficient for buildings and processes that require cooling on very cold days. Therefore, what is needed is a cooling system and method that is effective on cold days and that does not require the inefficient, power intensive use of mechanical cooling.